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Re: Evolution of tyrannosauroid bite power

On Sun, Dec 30, 2007 at 04:44:06PM -0500, ptnorton scripsit:
> Don Ohmes wrote, in part:
> *Snip*
>> that a relatively large and powerful head was absolutely essential to life 
>> style, because it is relative >arm length that decreased (even while the 
>> arms remained strong), rather than relative head size.
> "Strong" is a relative term. Carpenter and Smith (2001) estimated the 
> maximum force capable of being generated by the M. biceps in T. rex to be 
> about 1955 N, or about 199 kg (440 pounds) per arm. Although grasping 900 
> pounds or so may sound "strong" in absolute terms, in relative terms 
> (assuming a 6000-7000 kg mass for an adult T. rex) it equates to me (a 200 
> pound person) being able to curl about 12 pounds, using both arms. I think 
> the question is not why T. rex arms were so "strong", but why were they so 
> weak?

Because there wasn't much selective pressure acting on them?

T. rex was pretty clearly using some combination of legs and bite to
capture prey.  The arms were not centrally involved.  (Or if they were
involved, it was in a way -- position sensors for a 'grappled' prey
animal, say -- that didn't select for strength.)

Some other large theropod that used its arms for primary prey capture --
consider a hypothetical case where something secondarily flightless
uses the wing-raising stroke to claw vertically and hamstring prey --
*and* needed a powerful bite (it's eating members of Thyreophora; it has
to eat really fast before the local T. rex pack steals the kill; it has
to get through half a foot of sauropod hide; etc.) could have powerful
arms and a powerful bite.

It wouldn't be likely to have *as* powerful a bite as a T. rex, because
everything is tradeoffs and there's a finite amount of muscle mass to
distribute over the organism, but there's nothing deterministic about
'powerful bite, puny arms' in theropods.

-- Graydon